In the production of molded polyurethane products from flowable reactants to obtain an end product which is free from defects, the flow rates of the components which depend on various factors such as viscosity, temperature, pressure, must be monitored continuously. As these factors are limited by variations in the structure of the components and by tolerances in the mode of operation of the mixing device, it is very difficult to keep them completely constant. It is therefore necessary to keep the flow rate constant as these factors change.
With the known devices, the metering pumps are tested by allowing the quantity conveyed per unit time to flow out at a certain pressure and then weighing it at various adjustments of the pump control path. According to the pump characteristics determined in this way (which are based on the dependence of the quantity conveyed Q as a function (f) of the control path of the pumps) the mixing device is adjusted with respect to the correct, prescribed stoichiometric ratio. This occurs before production commences. The observance of these predetermined values cannot be checked during production. An entire time-consuming and expensive test run therefore must be repeated at certain intervals to give sufficient reliability in production. This test method is also accompanied by a large unreliability factor since the weight is measured only under atmospheric pressure. If one component contains a blowing agent, the blowing agent can escape under certain circumstances due to its low boiling point.
Attempts have been made to divide at least one component stream continuously into a main stream and a subsidiary stream, the subsidiary stream being brought to the same operating pressure as the main stream just before it is introduced into the mixing zone. The volumetric rate of flow of the subsidiary stream is then measured. The quantity by volume of the subsidiary stream which has passed through during a predetermined time interval is determined by measurement. The quantity by weight which has passed through during the same predetermined time interval is determined by weighing. The density of the component is determined as a quotient of the quantity by weight and the quantity by volume. The quantitative rate of flow of the main stream of the component is determined as the product of the density of the component and the volumetric rate of flow of the main stream. This mode of operation is very complex not only with regard to monitoring but also with regard to the apparatus.
The object of the invention is to provide a method and a device for the reliable, automatic regulation and monitoring of the rates of flow during the injection and circulation at predetermined pressures.